In recent years, application of a reconfigurable optical add drop multiplexing (ROADM) device to a submarine cable system has been considered. The ROADM device is able to re-configure connection between lines after an operation is started. Functions of the ROADM device are implemented by a wavelength selective switch (hereinafter, referred to as “WSS”) having a demultiplexing function, a switching function, and a multiplexing function.
FIG. 15 is a block diagram illustrating a configuration of a general ROADM device 900. The ROADM device 900 includes branching units 11 and 12, a wavelength selection unit 13, and a multiplexing unit 15. A trunk signal 101 and a branch signal 103 are input to the ROADM device 900. The ROADM device 900 generates and outputs a trunk signal 102 and a branch signal 104 from an optical signal included in the trunk signal 101 and the branch signal 103.
The trunk signal 101 is a signal obtained by wavelength-multiplexing an optical signal of a wavelength band A and an optical signal of a wavelength band B. The branch signal 103 is a signal obtained by wavelength-multiplexing an optical signal of a wavelength band C and an optical signal of a wavelength band D. In the following description, each of the optical signals is described by using a wavelength band of the optical signal. For example, an optical signal of the wavelength A is described as a signal A, and an optical signal obtained by wavelength-multiplexing an optical signal of the wavelength band A and an optical signal of the wavelength band B is described as a signal AB. Therefore, the trunk signal 101 may be described as the signal AB, and the branch signal 103 may be described as a signal CD. Further, in the drawings, the signal AB and the signal CD are described as “(AB)” and “(CD)”. The same definition is also applied to signals of other wavelength bands. Note that in the present application, arrows in the drawings indicate only propagation directions of signals associated with description, and do not limit propagation directions of other signals.
The signal D is a dummy signal for use in the branch signals 103 and 104. The signal D is transmitted in order to set a total optical power of a transmission path on the branch side within a fixed range. The signal D does not have to carry information to be transmitted. By using the signal D, it is possible to share a specification of an optical repeater amplifier for use in a submarine cable system.
The branching unit 11 branches the input trunk signal 101 (signal AB) into two signals, and outputs the two signals to the wavelength selection unit 13 and the multiplexing unit 15. The branching unit 12 branches the input branch signal 103 (signal CD) into two signals, and outputs the two signals to the wavelength selection unit 13 and the multiplexing unit 15. A WSS is used in the wavelength selection unit 13. The multiplexing unit 15 outputs the trunk signal 102 (signal AC) from the signal AB and the signal CD. The wavelength section unit 13 outputs the branch signal 104 (signal BD) from the signal AB and the signal CD.
In association with the present invention, PTLs 1 and 2 describe an optical cross-connect device using a WSS.